Numerous hydrocarbon conversion processes are widely used to alter the structure or properties of hydrocarbon streams. Such processes include isomerization from straight chain paraffinic or olefinic hydrocarbons to more highly branched hydrocarbons, dehydrogenation for producing olefinic or aromatic compounds, reforming to produce aromatics and motor fuels, alkylation to produce commodity chemicals and motor fuels, transalkylation, and others.
Many such processes use catalysts to promote hydrocarbon conversion reactions. These catalysts tend to deactivate for a variety of reasons, including the deposition of carbonaceous material or coke upon the catalyst, sintering or agglomeration or poisoning of catalytic metals on the catalyst, and/or loss of catalytic metal promoters such as halogens. Consequently, these catalysts are typically reactivated in a process called regeneration. Reactivation can thus include, for example, removing coke from the catalyst by burning (combustion), redispersing catalytic metals such as platinum on the catalyst, oxidizing such catalytic metals, reducing such catalytic metals, replenishing catalytic promoters such as halogens on the catalyst, and drying the catalyst.
While catalyst regeneration can be conducted in fixed catalyst beds, it is commonly carried out in a moving bed regeneration zone that is associated with a moving bed reaction zone. Fresh catalyst particles are fed to a reaction zone, which may be comprised of several reactors, and the particles flow through the zone by gravity. Catalyst is withdrawn from the bottom of the reaction zone and transported to a regeneration zone where a regeneration process consisting of one or more steps is used to regenerate the catalyst to restore its full reaction promoting ability. Catalyst flows by gravity through the various regeneration steps and then is withdrawn from the regeneration zone and furnished to the reaction zone. Movement of catalyst through the zones is often referred to as continuous though, in practice, it is semicontinuous. By semicontinuous movement is meant the repeated transfer of relatively small amounts of catalyst at closely spaced points in time. For example, one batch per minute may be withdrawn from the bottom of a regeneration zone and withdrawal may take one-half minute, that is, catalyst will flow for one-half minute. If the inventory in the regeneration zone is large, the catalyst bed may be considered to be continuously moving. A moving bed system has the advantage of maintaining production while the catalyst is removed or replaced. U.S. Pat. Nos. 5,837,636 and 6,117,809 describe moving bed regeneration zones where coke combustion, metal redispersion, metal oxidation, metal reduction, promoter addition, and catalyst drying occur.
One of the problems during regeneration of halogen-containing catalysts is loss of halogen from the catalyst. This happens when catalyst particles are contacted with gases that, while regenerating the catalyst particles, tend also to remove halogen from the catalyst particles. Apart from any adverse effect of halogen loss on catalytic activity, venting of a gas stream containing a halogen from the process poses an environmental concern. The environmental concern can be abated either by scrubbing the vent gas with an aqueous, basic solution or by adsorbing the halogen from the vent gas on an adsorbent. The halogen can also be adsorbed on the catalyst particles themselves as disclosed in U.S. Pat. Nos. 5,837,636 and 6,117,809, for example.
Adsorption of the halogen on the catalyst particles is advantageous since it does not involve the expense of a separate adsorbent and its associated vessel(s) and equipment. However, in some circumstances adsorbing the halogen on catalyst particles that circulate through moving bed reaction and regeneration zones is complicated and expensive.
A process is sought that removes halogens from gas streams vented from processes for regenerating catalyst particles.